Ptfe polymer-based sliding material having fillers which improve the tribological properties

ABSTRACT

The invention relates to a PTFE polymer-based sliding material having fillers which improve the tribological properties, wherein the fillers comprise at least one phosphate, in particular calcium phosphate, calcium pyrophosphate, magnesium phosphate, magnesium pyrophosphate, lithium phosphate, hydroxyapatite or combinations thereof, and at least one metal sulfide, wherein the fraction of the metal sulfide is &gt;2% by volume. The invention also relates to uses of said sliding material.

The invention relates to a PTFE polymer-based sliding material having fillers which improve the tribological properties.

In the prior art, a variety of sliding materials are known, which are, for example, PTFE-based and use a variety of fillers.

For example, DE 10 2011 077 008 A1 describes a PTFE-based sliding bearing composite material, wherein fillers comprising thermoplastics and/or thermosets and further tribologically active substances are present together in compounded form in the polymer base of the sliding layer material.

Furthermore, EP 2 563 590 B1 describes a fluoropolymer-based sliding bearing material, for example PTFE-based, wherein 5-25% by volume boron nitride and 1-15% by volume mixed phase oxide pigments are added to the fluoropolymer, which is intended to improve wear resistance.

EP 2 316 707 A1 and EP 1 647 574 A1 describe a PTFE-based sliding bearing material with barium sulfate, phosphate and an amount of 0.1 to 2% of metal sulfide.

Finally, WO 2016/170742 A1 discloses a material based on PTFE and 10-40% phosphates.

Despite the variety of sliding materials described in the prior art, there remains a need for a sliding material that is wear-resistant and suitable for both dry-running and lubricated rotational and axial applications.

It is an object of the invention to improve the tribological properties and, in particular, at the same time the wear resistance of PTFE polymer-based sliding materials in the various applications.

This problem is solved by a PTFE polymer-based sliding material having the features of claim 1. Advantageous developments are described in claims 2 to 12.

In addition, the invention relates to a PTFE-based sliding bearing composite material according to claim 13 and a sliding bearing element according to claim 14.

It has surprisingly been found that the wear resistance of the sliding material is significantly improved by the addition of at least one phosphate together with at least more than 2% by volume of metal sulfide, being used as the phosphate in particular a calcium phosphate, calcium pyrophosphate, magnesium phosphate, magnesium pyrophosphate, lithium phosphate, hydroxyapatite or combinations thereof, as fillers for the PTFE polymer base. This can already be achieved without the addition of further fillers. Particularly preferred is the use of a calcium phosphate. As calcium phosphate, for example, pentacalcium phosphate, pentacalcium hydroxy orthophosphate (hydroxyapatite), tricalcium bis (orthophosphate), dicalcium pyrophosphate or calcium hydrogen orthophosphate can be used.

Particularly advantageous properties result when the proportion of phosphates in total is 1 to 30% by volume, in particular 1 to 25% by volume and in particular 1 to 20% by volume of the sliding material. Particular preference is given to proportions of between 10 and 15% by volume.

Particularly advantageous properties result when the proportion of phosphates in total is 1 to 30% by volume, in particular 1 to 25% by volume and in particular 1 to 20% by volume of the sliding material. Particular preference is given to proportions of between 10 and 15% by volume. The proportion is at least 1% by volume, preferably at least 3% by volume, preferably at least 5% by volume, more preferably at least 10% by volume, more preferably at least 12% by volume and more preferably at least 15% by volume. More preferably, the proportion is at most 30% by volume, more preferably at most 25% by volume, further preferably at most 20% by volume, and finally more preferably at most 15% by volume.

For this purpose, the sliding material contains in addition to the phosphate one or more metal sulfides in a proportion of more than 2% by volume, preferably more than 3% by volume and more preferably more than 5% by volume, preferably more than 7% by volume and more preferably more than 9% by volume, and more preferably 10% by volume or more than 10% by volume. More preferably, the proportion of metal sulfide is at most 30% by volume, preferably at most 20% by volume and more preferably at most 15% by volume.

In addition to the phosphate used, the metal sulfide used with a proportion of more than 2% by volume further improves the tribological properties. As metal sulfides, preferably at least one of the group SnS₂, MoS₂, Bi₂S₃, WS₂, ZnS and/or CuS and combinations are used.

The sliding material according to the invention is preferably lead-free.

The present sliding material is suitable in particular for uses outside the motor in the automotive sector, in particular in common rail pumps, shock absorbers, gearboxes and steering systems, in particular in rotational and axial applications. In such applications, there is a desire to use lead-free sliding materials. The present sliding material now provides a material which has improved wear resistance, in particular with equal or even improved friction behavior.

In the sliding material according to the invention, a PTFE base is used as the polymer base for the polymer matrix. This does not necessarily mean that the entire polymer base must be made of PTFE. Proportions of up to 30% by volume of the polymer fraction, in particular up to 20% by volume and preferably only up to 10% by volume, of the PTFE polymer base may be replaced by other polymers and combinations thereof, in particular PVDF, PFA, FEP, ECTFE, ETFE. Preferably, the entire PTFE polymer base consists of up to 100% by volume of PTFE.

The ratio of the volumes of phosphate to metal sulfide is preferably 6:1 to 1:2, more preferably 4:1 to 1:1.

In particular, one or more phosphates, for example calcium phosphate, may also be combined with two or more metal sulfides.

The volume fractions are determined by means of weight and density.

The sliding material may preferably comprise further secondary fillers, in particular further solid lubricants, in particular BaSO₄, lithophones and/or fluorides, especially calcium fluoride, and/or pigments and combinations of the aforementioned secondary fillers. As a result, the wear properties are further improved.

In addition, other tertiary fillers, in particular carbon fibers, glass fibers, polymer fibers (in particular aramid fibers); and/or solid lubricants, in particular graphite, carbon black, BN and/or plastic particles, such as PFPE, aramid (PPTA), PPSO₂, PI and PAI particles, polyacrylate particles (PAR), PBA particles, PBI particles; and/or metal oxides, in particular Fe₂O₃, Al₂O₃, SiO₂, CrO₂, TiO₂, CuO, MgO, ZnO; and/or hard material particles, in particular ceramic particles, such as SiC, Si₃N₄, BC, cubic BN; and/or fluorides, such as in particular NaF, AlF₃; and/or sheet silicates, in particular kaolin, mica, wollastonite, talc, silicic acid; and/or metallic fine powders, in particular bronze and bismuth; and/or pigments or mixed phase oxide pigments, in particular Co—Al, Cr—Sb—Ti, Co—Ti, Fe—Al, Mn—Fe or Co—Cr may be contained in the sliding material.

In this case, the proportion of secondary fillers is preferably 0 to 30% by volume, in particular 1 to 20% by volume and in particular 1 to 10% by volume of the sliding material. The proportion of tertiary fillers in the sliding material may be 0 to 10% by volume of the sliding material.

Furthermore, it is particularly preferred that the PTFE polymer base constitutes 50 to 95% by volume, in particular 60 to 95% by volume and in particular 70 to 90% by volume of the sliding material. In this way, the properties of the PTFE material can affect the sliding material in a sufficient manner.

The sliding material can serve for producing a sliding bearing composite material with a metallic support layer, in particular made of steel or bronze, optionally with a porous support layer, in particular made of sintered bronze and with a sliding material filling the pores of the support layer or bearing layer of the type described above. For this purpose, the PTFE based sliding material having the fillers is preferably in paste form. The pasty sliding material is then applied to the porous layer and impregnated into the pores via a rolling process and then sintered.

The sliding bearing composite material can be used as a sliding bearing element, wherein a sliding bearing element can be, for example, a sliding strip, a sliding block, a sliding pad and a plain bearing shell, a slide bearing bush or a slide bearing collar bush.

Sliding elements may typically also be rolled cylindrical bushes or half-shell sliding bearing elements made with the sliding bearing composite. The sliding bearing composite material can also be used for the production of flanged bushes or cup bushes or planar and spherical sliding bearing elements.

Below are listed several examples and references.

Example Fillers Polymer matrix (Comparison) 20% by volume of ZnS PTFE (remainder) 1 (Reference) 20% by volume of calcium PTFE ( remainder ) 2 phosphate (Comparison) 20 by volume of Bi₂S₃ PTFE (remainder) 3 (Comparison) 20% by volume of SnS₂ PTFE (remainder) 4 (Comparison) 20% by volume of WS₂ PTFE (remainder) 5 6 15% by volume of calcium PTFE (remainder) phosphate 10% by volume of Bi₂S₃ 7 15% by volume of calcium PTFE (remainder) phosphate 10% by volume of SnS₂ 8 15% by volume of calcium PTFE (remainder) phosphate 10% by volume of WS₂ 9 10% by volume of calcium PTFE (remainder) phosphate 10% by volume of Bi₂S₃ 10% by volume of BaSO₄

The calcium phosphate used in the examples is in all cases tri-calcium phosphate Ca₃(PO₄)₂.

In the drawings:

FIG. 1 shows a schematic sectional view of a metal/plastic sliding bearing composite material according to the invention;

FIG. 2 shows measurement results of wear values of sliding bearing members made of the above sliding materials;

FIG. 3 shows measurement results of friction coefficients of sliding bearing members made of the above sliding materials.

FIG. 1 shows a schematic sectional view of a sliding bearing composite material 2 according to the invention with a metallic support layer 4, typically of steel, and with a porous carrier layer 6. The porous carrier layer 6 is formed by a sintered layer of bronze-based metallic particles 7. The particles of the carrier layer 6 form coherent macroscopic cavities (which are not shown to scale), in which a polymer-based sliding material 8 according to the invention is impregnated. The sliding material 8 fills the pores of the support layer 6 substantially completely. The matrix-forming polymer component of the sliding layer material 8 is based on PTFE in the sense defined above. The polymer component is advantageously made of PTFE. The sliding layer material 8 further comprises fillers incorporated in the matrix-forming polymer component, namely one or more phosphates and one or more metal sulfides. Other fillers in addition to phosphates and metal sulfides may prove to be advantageous and are also not shown. In particular, the above-mentioned secondary and/or tertiary fillers are suitable.

FIG. 2 shows the result of wear measurements and FIG. 3 of friction coefficient measurements on sliding bearing elements, which were measured with sliding bearing composite materials using sliding materials of the above examples 1 to 9. The sliding bearing composites comprise a metallic support layer of steel, a porous support layer of the type described above, and a polymer-based sliding material filling the pores of the support layer. The examined sliding bearing elements differ only with regard to the composition of the sliding material.

The wear resistance and friction coefficients were determined in a rotation test with a sliding speed of 2 m/s under a load of 0.75 MPa. The test parameters are given in the table below.

Test Test parameters parameters Lubrication Dry run condition Counter body 100Cr6 Load 0.75 MPA Sliding speed 2 m/s Test time 15 h

For this purpose, a bush was produced as a sliding bearing element having an inner diameter of 20 mm and an outer diameter of 23 mm and 15 mm width, comprising the sliding material according to the above examples 1 to 9.

Example 2, which contains only calcium phosphate as a filler, serves as reference example, to the measured values of which the other examples are related. Examples 1 and 3 to 5 are not comparative examples according to the invention. Examples 6 to 9 show sliding materials according to the invention.

The sliding materials to which calcium phosphate and a metal sulfide have been added as fillers and which thus correspond to the invention show a significantly and surprisingly lower wear than the comparative examples and the reference example.

The wear resistance can be further improved by targeted combinations of calcium phosphate with metal sulfides as further solid lubricants (primary fillers), such as Bi₂S₃ (Example 6), SnS₂ (Example 7) or WS₂ (Example 8). Furthermore, a mixture of phosphate and primary fillers (here Bi₂S₃) and secondary fillers, here BaSO₄ as a solid lubricant, were investigated, which further improves the wear values (s. Example 9). Especially the examples 8 and 9 show a more than 50% lower wear than the references with consistently good friction coefficients. 

1. PTFE polymer-based sliding material having fillers which improve the tribological properties, characterized in that the fillers comprise at least one phosphate and at least one metal sulfide as a primary filler and wherein the proportion of the at least one metal sulfides is at least >2% by volume.
 2. The PTFE polymer-based material according to claim 1, characterized in that the at least one phosphate calcium phosphate, calcium pyrophosphate, magnesium phosphate, magnesium pyrophosphate, lithium phosphate, hydroxyapatite or combinations thereof.
 3. The PTFE polymer-based siding material according to claim 1, characterized in that the at least one metal sulfide comprises SnS₂, MoS₅, Bi₂S₃, WS₂, ZnS or CuS and combinations thereof.
 4. The PTFE polymer-based sliding material according to claim 1, characterized in that the PTFE polymer-base sliding material comprises a PTFE polymer-base having at least 70% by volume, including at least 80% by volume, at least 90% by volume and 100% by volume of PTFE, and also including proportions of up to 30% by volume of a polymer fraction, including up to 20% by volume, and or up to 10% by volume, of the PTFE polymer-base is replaced by other polymers and combinations thereof, including PVDF, PFA, FEP, ECTFE, ETFE.
 5. The PTFE polymer-based sliding material according to claim 1, characterized in that the proportion of the at least one phosphates in total is 1 to 30% by volume, or 1 to 25% by volume, or 1 to 20% by volume of the PTFE polymer-based sliding material.
 6. The PTFE polymer-based sliding material according to claim 1, characterized in that the proportion of the at least one metal sulfides as fillers in total is more than 3% by volume, or more than 5% by volume, or more than 7% by volume, or more than 9% by volume, or 10% by volume, or more than 10% by volume, or at most 30% by volume, or at most 20% by volume, or at most 15% by volume of the PTFE polymer-based sliding material.
 7. The PTFE polymer-based sliding material according to claim 1, characterized in that the ratio of volumes of the at least one phosphate to the at least one metal sulfide is 6:1 to 1:2, including 4:1 to 1:1.
 8. The PTFE polymer-based sliding material according to claim 1, characterized in that the PTFE polymer-based sliding material comprises further secondary fillers, including further solid lubricants, or, BaSO₄, Lithophones and/or fluorides, or calcium fluoride, and/or pigments and combinations of the further secondary fillers.
 9. The PTFE polymer-based sliding material according to claim 7, characterized in that the proportion of the further secondary fillers is 0 to 30% by volume, or 1 to 20% by volume, or 1 to 10% by volume of the PTFE polymer-based sliding material.
 10. The PTFE polymer-based sliding material according to claim 1, characterized in that further tertiary fillers are carbon fibers, glass fibers, polymer fibers (in particular aramid fibers); and/or solid lubricants, including graphite, carbon black, BN and/or plastic particles, such as PFPE, aramid (PPTA), PPSO₂, PI and PAI particles, polyacrylate particles (PAR), PBA particles, PBI particles; and/or metal oxides, including Fe₂O₃, Al₂O₃, SiO₂, CrO₂, TiO₂, CuO, MgO, ZnO; and/or hard material particles, including ceramic particles, such as SiC, Si₃N₄, BC, cubic BN; and/or fluorides, such as in particular NaF, AlF₃; and/or sheet silicates, including kaolin, mica, wollastonite, talc, silicic acid; and/or metallic fine powders, in particular bronze and bismuth; and/or pigments or mixed phase oxide pigments, including Co—Al, Cr-13 Sb—Ti, Co—Ti, Fe—Al, Mn—Fe or Co—Cr.
 11. The PTFE polymer-based siding material according to claim 9, characterized in that the proportion of further tertiary fillers substances is 0 to 15% by volume, including 0 to 10% by volume of the PTFE polymer-based sliding material.
 12. The PTFE polymer-based sliding material according to claim 1, characterized in that the PTFE polymer-based sliding material comprises a PTFE polymer base that constitutes 50 to 95% by volume, including 60 to 95% by volume, or 70 to 90% by volume of the PTFE polymer-based sliding material.
 13. A PTFE polymer-based sliding bearing composite material with a metallic support layer, including steel or bronze, including with a porous support layer, including being made of bronze, and with a sliding material filling, pores of the metallic support layer according to claim
 1. 14. A sliding bearing element made of the PTFE polymer-based sliding bearing composite material according to claim
 13. 15. The sliding bearing element according to claim 14, characterized in that the sliding bearing element is a sliding strip, a sliding shoe or sliding pad or a sliding bearing shell, a slide bearing bush or slide collar bush.
 16. The PTFE polymer-based sliding material according to claim 2, characterized in that the at least one metal sulfide comprises SnS₂, MoS_(s), Bi₂S₃, WS₂, ZnS or CuS and combinations thereof.
 17. The PTFE polymer-based sliding material according to claim 2, characterized in that the PTFE polymer-base sliding material comprises a PTFE polymer-base having at least 70% by volume, including at least 80% by volume, at least 90% by volume and 100% by volume of PTFE, and also including proportions of up to 30% by volume of a polymer fraction, including up to 20% by volume, and or up to 10% by volume of the PTFE polymer-base is replaced by other polymers and combinations thereof, including PVDF, PFA, FEP, ECTFE, ETFE.
 18. The PTFE polymer-based sliding material according to claim 2, characterized in that the proportion of the at least one phosphates in total is 1 to 30% by volume, or 1 to 25% by volume, or 1 to 20% by volume of the PTFE polymer-based sliding material.
 19. The PTFE polymer-based sliding material according to claim 2, characterized in that the proportion of the at least one metal sulfides as fillers in total is more than 3% by volume, or more than 5% by volume, or more than 7% by volume, or more than 9% by volume, or 10% by volume, or more than 10% by volume, or at most 30% by volume, or at most 20% by volume, or at most 15% by volume of the PTFE polymer-based sliding material.
 20. The PTFE polymer-based sliding material according to claim 2, characterized in that the ratio of volumes of the at least one phosphate to the at least one metal sulfide is 6:1 to 1:2, including 4:1 to 1:1. 